The present invention relates to a ball screw actuator for aircraft control surfaces. Aircraft control surfaces are commanded by appropriate actuators able to move them between two extreme positions, in such a way as to make the surfaces assume operative configurations suitable for specific phases of flight. For example, actuators are used to command the flaps installed on the trailing edge of a wing or to control the elevators mounted on the tail empennage.
Ball screw actuators are known which are constituted by a lead screw set in rotation about its own longitudinal axis by a motor and associated to a tubular body, known in the specific art as lead nut, mounted coaxially on the screw itself. The lead nut is cinematically connected to the control surface whilst the screw is mounted integrally on the load bearing structure of the aircraft. The lead nut, which is prevented from rotating, has on its own inner surface appropriate seats for a plurality of balls able to rotate on themselves and to slide in the throat of the thread of the lead nut. The rotation of the lead screw determines, by means of the coupling provided by the screws, the sliding of the lead nut along the longitudinal axis of the screw and the consequent motion of the control surface.
Also known are actuators in which the screw is connected to the control surface and translates, whilst the lead nut is connected to the structure and rotates. Since in the aeronautical field it is necessary to guarantee a very high safety margin to the structures, or put the aircraft at risk of crashing, prior art actuators are provided with particular devices to prevent control surfaces from becoming uncontrollable due to a failure in the lead nut or lead screw.
The loss of the balls of the lead nut or the rupture thereof leads to the loss of the structural continuity between the controls and the controlling surfaces, since the lead nut is free to slide relative to the screw. In this situation, the surfaces are free to move under the action of aerodynamic and inertial forces and the aircraft is absolutely uncontrollable.
To overcome this drawback, known actuators have been built which comprise an auxiliary lead nut, connected to the main one, which becomes operative when a malfunction occurs in the main lead nut itself. The auxiliary lead nut serves as a safety device.
For instance, auxiliary ball lead nuts are known which are structurally similar to the primary ones. Although this type of solution assures control even after the rupture of the main lead nut, it should be noted that auxiliary lead nuts can be subject to the same type of failure as the primary ones.
Also known are inverse thread auxiliary lead nuts constituted by a tubular portion connected to the main lead nut which has on its own inner surface, facing the lead nut, a thread with reversed shape relative to the thread of the screw.
During the proper operation of the main lead nut, the balls maintain the reversed thread of the auxiliary lead nut at a determined distance from the thread of the screw and the tubular portion of the is perfectly coaxial to the longitudinal axis of the screw.
As a result of the loss of the balls caused by a failure, both lead nuts lose the coaxial positioning relative to the shaft of the screw and the inverse thread of the auxiliary lead nut is engaged in the throat of the thread of the lead nut.
A first type of inverse thread auxiliary lead nut is made of low friction material which slides in the throat of the thread of the screw and serves the function of the balls for a certain fraction of the required working life. The main drawback of such a solution is that the duration of the low friction inverse thread cannot be estimated correctly; the thread can be damaged rapidly and lead to the lack of structural connection between the aforementioned components.
Auxiliary lead nuts are known with high friction coefficient which causes the seizure of the auxiliary lead nut on the screw and the consequent locking of the main lead nut and of the control surfaces connected thereto. The control surfaces are no longer controllable, but remain motionless in a determined position, allowing in any case to control the aircraft to a landing.
The seizure of the inverse thread on the screw occurs during the rotation thereof. The torque imparted to the screw by the motor is contrasted by the friction torque imparted by the inverse thread under the action of the external load. Consequently, the load at which the seizure occurs may be very high and not included in the flight envelope of the aircraft.
Also known from the document U.S. Pat. No. 6,467,363 is a device for blocking a ball screw actuator, which comprises a sensor mechanism, able to detect a malfunction of the primary lead nut, and means for blocking the screw relative to the primary lead nut in case of failure. The locking means are defined by a pair of disk-shaped elements which, activated by the sensor mechanism, are thrust by respective springs between the throat of the thread of the screw and an inner surface of a containment body of the screw itself. This device is formed by a multiplicity of elements easily subject to rupture which cannot assure the necessary safety margin.
Lastly, the document U.S. Pat. No. 4,644,811 has a ball screw actuator provided with a free wheel end stop device. The actuator comprises a ball screw threaded on a predominant portion except at one of its ends. The screw is inserted in a main lead nut, provided with balls engaged on the thread of the screw, and in a secondary lead nut, integral with the main one and driven thereby. The secondary lead nut is coupled with the main one by means of a retaining pin and a thrust bearing interposed axially.
When the main lead nut exits the threaded portion of the screw and tends to continue its own run, the secondary lead nut is engaged on the thread of screw and tends to rotate with the screw itself breaking the retaining pin. This device allows the lock the lead nut only at the end of the run.